This invention relates generally to imaging systems and, more particularly, to methods and apparatus for retrospectively reconstructing an image using a region of interest specified by an imaging system operator.
Imaging systems include a source that emits signals (including but not limited to x-ray, radio frequency, or sonar signals), and the signals are directed toward an object to be imaged. The emitted signals and the interposed object interact to produce a response that is received by one or more detectors. The imaging system then processes the detected response signals to generate an image of the object.
For example, in computed tomography (CT) imaging, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d,which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one-fan-beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as improved image quality and better control of contrast.
It is known to use an imaging system, e.g. a CT imaging system, to retrospectively reconstruct images, e.g. axial images, from raw scan data. Retrospectively reconstructed images can differ in various respects from original scan images. For example, axial slices can be reconstructed closer together or farther apart than those of the original scan, and the scan field of view can be reduced to concentrate reconstruction within a smaller scan region in order to resolve more detail.
When prescribing retrospective image reconstruction, an operator enters a series of parameters into the imaging system in order to specify image reconstruction characteristics such as a new field of view. Having to determine and enter sequences of parameters, however, can increase chance for error.
It would be desirable to allow an imaging system operator to select parameters specifying retrospective image reconstruction without having to determine and enter a lengthy parameter sequence. It also would be desirable to provide the operator with a way to use a selection of a region of interest within a scan to specify retrospective image reconstruction.
There is therefore provided, in one embodiment, a method for reconstructing an image from imaging data using an imaging system configured to accept input from an imaging system operator, the method including the steps of generating a first model from the imaging data; accepting as input an operator-specified region of interest based on the first model; and generating a second model from the imaging data based on the specified region of interest.
The above-described method simplifies entry of parameters for retrospective image reconstruction by using the operator-entered region of interest as input for automated parameter selection.